摘要:
桐柏造山带中秦岭群中出露有志留纪麻粒岩,然而其原岩形成时代和变质年龄却没有得到有效的约束。本文对北桐柏秦岭群中长英质麻粒岩中锆石开展了详细的形貌学、阴极发光图像、微量元素以及U-Pb年代学研究。LA-ICP-MS锆石U-Pb定年结果表明继承碎屑锆石给出了2.7~0.55 Ga的年龄,岩浆锆石给出了478±7 Ma(MSWD=0.73)的206Pb/238U加权平均年龄,代表了桐柏长英质麻粒岩原岩的形成年龄。变质锆石区域获得了437±7 Ma(MSWD=1.3,n=10)的206Pb/238U加权平均年龄。结合锆石微量元素特征和前人研究结果表明437±7 Ma可能代表了前进变质阶段的年龄。而最外层的增生边给出了406±8 Ma(MSWD=0.72,n=4)的206Pb/238U加权平均年龄,代表了退变质年龄。我们认为在480 Ma时,商丹洋已开始向北俯冲,导致480 Ma左右的岩浆作用,由于持续的俯冲消减,在440~420 Ma时,洋中脊与弧交汇,发生洋脊俯冲导致志留纪麻粒岩相变质作用以及岩浆作用的发生。
Abstract:
Granulite-facies metamorphic rocks in the Tongbai orogen have great implications for the evolution of the whole Qinling-Tongbai-Dabie orogenic belt. However, the protolith age and granulite-facies metamorphic timing of the granulites have not been well constrained yet. In this paper, CL imaging, trace element analysis and U-Pb dating of zircon were undertaken for felsic granulite from Tongbai orogen. U-Pb ages of the inherited detrial zircons and the magmatic zircons are 2.7Ga-0.55Ga and 478±7Ma (MSWD=0.73), respectively, which represent the formation age of the protolith. The metamorphic zircon rim yield a weighted mean age of 437 ±7Ma (MSWD=1.3, n=10),which represent the metamorphic age. Considering the trace-element characteristics of zir-cons as well as former studies, the age of 437±7 Ma was suggested as the prograde metamorphic age. The outer overgrowth rim yield a weighted mean age of 406 ±8 Ma (MSWD=0.72, n=4), suggesting retrograde metamor-phism. Available results indicate that the northward subduction of the Prototethyan Shangdan oceanic crust pro-duced a series of magmatic pulses and metamorphism at ca. 480 Ma in the North Qinling-Tongbai orogen. At
440420 Ma, a ridge is interpreted to have entered the subduction realm, resulting granulite-facies metamorphism and coeval magmatism.
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